In an electrophotographic image forming apparatus (hereinafter also referred to as an electrophotographic apparatus) used, for example, as a copying machine, a printer, or a facsimile apparatus, an image is formed by way of the following electrophotographic process. At first, a surface of an electrophotographic photoreceptor (hereinafter also referred to simply as a photoreceptor) provided in the apparatus is charged uniformly to a predetermined potential by a charger, and exposed to light in accordance with image information by exposure means so that an electrostatic latent images are formed. The formed electrostatic latent image is developed by use of a developer containing a toner supplied from development means so that a toner image which is a visible image is formed. The formed toner image is transferred by transferring means from the surface of the photoreceptor onto a transfer member, for example, recording paper, and fixed thereto by fixing means. Furthermore, the surface of the photoreceptor onto which the toner image has been transferred is cleaned by cleaning means to remove a toner remained on the photoreceptor surface without being transferred on to the transfer member, and foreign objects such as paper powder of the recording paper which are deposited during transfer and remained on the photoreceptor surface. After this, the surface of the photoreceptor is charge-eliminated by a charge eliminator so that the electrostatic latent image on the photoreceptor surface is eliminated.
An electrophotographic photoreceptor used in such an electrophotographic process is constituted by laminating a photosensitive layer containing a photoconductive material on an conductive substrate. Conventionally, as the electrophotographic photoreceptor, an electrophotographic photoreceptor using an inorganic photoconductive material (hereinafter referred to as an inorganic photoreceptor) has been used. Typical inorganic photoreceptor includes a selenium-series photoreceptor using a layer comprising an amorphous selenium (a-Se) or an amorphous selenium arsenide (a-AsSe) as a photosensitive layer, a zinc oxide-series or cadmium sulfide-series photoreceptor using zinc oxide (chemical formula: ZnO) or cadmium sulfide (chemical formula: CdS) together with a sensitizer such as a dye being dispersed in a resin as the photosensitive layer, and an amorphous silicon-series photoreceptor (hereinafter referred to as a-Si photoreceptor) using a layer comprising amorphous silicone (a-Si) as a photosensitive layer.
However, the inorganic photoreceptor has the following drawbacks. The selenium-series photoreceptor and the cadmium sulfide-series photoreceptor have drawbacks in view of the heat resistance and the store stability. Further, since selenium and cadmium have toxicity to human bodies and environments, the photoreceptors using them have to be recovered and discarded properly after use. Further, the zinc oxide photoreceptor has a drawback that it has low sensitivity and low durability and is scarcely used at present. Further, the a-Si photoreceptor attracting attention as the inorganic photoreceptor with no public pollution has advantages such as high sensitive and high durability but since this is manufactured by using a plasma chemical vapor deposition method, the a-Si photoreceptor has such drawbacks that it is difficult to uniformly deposit the film of the photosensitive layer and that image defects are easily caused. Further, the a-Si photoreceptor also has a drawback of low productivity and high manufacturing cost.
In recent years, development has progressed for the photoconductive material used for the electrophotographic photoreceptor, and organic photoconductive materials (that is, Organic Photoconductor: abbreviated as: OPC) have been now used frequently instead of the inorganic photoconductive materials used so far. While the electrophotographic photoreceptor using the organic photoconductive material (hereinafter referred to as organic photoreceptor) involves some problems in view of the sensitivity, durability and stability to environment, it has various advantages compared with the inorganic photoreceptor in view of the toxicity, the production cost and the degree of freedom for the material design. Further, the organic photoreceptor also has an advantage that the photosensitive layer can be formed by an easy and inexpensive method typically represented by a dip coating method. Since the organic photoreceptor has such various advantages, it has now gradually been predominant in the electrophotographic photoreceptors. In response to demands of recent years for significant improvement of the sensitivity and durability, the organic photoreceptor has been increasingly used at present as the electrophotographic photoreceptor except for special cases.
Especially, the efficiency of organic photoreceptor is being significantly developed by the development of a function-separated electrophotographic photoreceptor of which charge generating function and charge transporting function are separately attained by different substances. In addition to the above-mentioned advantages of the organic photoreceptor, such a function-separated photoreceptor has an advantage that a latitude in selecting materials respectively for charge generating substance bearing the charge generating function and for charge transporting substance bearing the charge transporting function is broad and thus, a photoreceptor having any desired characteristics can be relatively readily produced.
The function-separated photoreceptor includes a lamination type and a single layer type. The function-separated photoreceptor of lamination type is provided with a photosensitive layer of lamination type constituted by lamination of a charge generating layer containing a charge generating substance and a charge transporting layer containing a charge transporting substance. The charge generating layer and the charge transporting layer are generally formed such that the charge generating substance and the charge transporting substance are respectively dispersed in binder resins which are the binding agent. Further, the function-separated photoreceptor of single layer type is provided with a photosensitive layer of single layer type formed by dispersing the charge generating substance and the charge transporting substance together in a binder resin.
A variety of substances has been heretofore investigated for the charge generating substances that may be used in the function-separated photoreceptor, including, for example, phthalocyanine pigments, squarylium-series dyes, azo pigments, perylene pigments, polycyclic quinone pigments, cyanine dyes, squaric acid dyes and pyrylium salt-series dyes, and various materials of good light fastness and good charge generating ability have been proposed.
On the other hand, various compounds are known for the charge transporting substances, including, for example, a pyrazoline compound (e.g., refer to Japanese Examined Patent Publication JP-B2 52-4188 (1977)), a hydrazone compound (e.g., refer to Japanese Unexamined Patent Publication JP-A 54-150128 (1979), Japanese Examined Patent Publication JP-B2 55-42380 (1980), and Japanese Unexamined Patent Publication JP-A 55-52063 (1980)), a triphenylamine compound (e.g., refer to Japanese Examined Patent Publication JP-B2 58-32372 (1983). and Japanese Unexamined Patent Publication JP-A 2-190862 (1990)) and a stilbene compound (e.g., refer to Japanese Unexamined Patent Publications JP-A 54-151955 (1979) and JP-A 58-198043 (1983)). Recently, pyrene derivatives, naphthalene derivatives and terphenyl derivatives that have a condensed polycyclic hydrocarbon structure as the center nucleus have been developed (e.g., refer to Japanese Unexamined Patent Publication JP-A 7-48324 (1995))
The charge transporting substance is required to be:    (1) stable to light and heat;    (2) stable to active substances such as ozone, nitrogen oxides (chemical formula: NOx) and nitric acid that may be generated in corona discharging on charging the photoreceptor surface;    (3) having good charge transporting ability;    (4) highly compatible with organic solvents and binder resins;    (5) easy to produce and inexpensive. However, the above-mentioned charge transporting substance does not satisfy all of these requirements at high level though partly satisfying some of these requirements.
Further, in recent years, electrophotographic apparatuses such as a digital copying machine and a printer have reduced size and increased operation speed, and higher sensitivity corresponding to the size reduction and operation speed increase has been demanded for the photoreceptor characteristic, and a particularly high charge transporting ability is demanded for the charge transporting substance. Further, in a high speed electrophotographic process, since the time from the exposure to the development is short, it has been demanded for a photoreceptor of excellent light responsiveness. In a case where the light responsiveness is low, that is, the decaying speed for the surface potential after exposure is slow, the residual potential increases and the photoreceptor is used repetitively in a state where the surface potential of the photoreceptor is not decayed sufficiently. Consequently, the surface charges at a portion to be eliminated are not eliminated sufficiently by exposure, in a consequence whereof negative results such as lowering of the image quality in the early stage are caused. Since the light responsiveness depends on the charge transporting ability of the charge transporting substance, in terms thereof, the charge transporting substance having higher charge transporting ability is demanded.
For the charge transporting substance that satisfies such requirements, proposed is an enamine compound having higher charge transporting ability than that of the above-mentioned charge transporting substance (e.g., refer to Japanese Unexamined Patent Publications JP-A 2-51162 (1990), JP-A 6-43674 (1994) and JP-A 10-69107 (1998)). Further, in another conventional art, in order to improve hole transporting ability of the photoreceptor, incorporation of polysilane and an enamine compound having a specified structure to a photosensitive layer is proposed ( e.g., refer to Japanese Unexamined Patent Publication JP-A 7-134430 (1995)).
Further, in the electrophotographic apparatus, since the above-mentioned operations of charging, exposure, development, transfer, cleaning, and charge elimination to the photoreceptor are conducted repetitively, the photoreceptor is required to be excellent in the durability to electrical and mechanical external forces in addition to high sensitivity and excellent light responsiveness. Specifically, it has been demanded that abrasion and flaw are not caused by friction with a cleaning member or the like to the surface layer of the photoreceptor and it is not degraded by deposition of active substance such as ozone and NOx generated upon electric discharge during the charged state.
One of indicators for evaluation of properties of materials, which are not limited to properties of the photoreceptor surface but properties at large, particularly for evaluation of mechanical property, is hardness. The hardness is defined as a stress brought from materials against indentation of an indenter. By use of this hardness in a physical parameter for learning properties of materials, an attempt is given to quantification of mechanical properties of such a film that constitutes the photoreceptor surface. As testing methods of measuring the hardness, for example, the scratch strength test, the pencil hardness test, and the Vickers hardness test are widely known.
However, all of these hardness tests have a problem in measuring the mechanical properties of the material which shows a complicated behavior combining plasticity, elasticity (including a delay component), and a creeping property as in the case of a film which is composed of organics. For example, the Vickers hardness indicates hardness evaluated by measuring a length of indentation produced on a film, but this reflects only plasticity of the film and thus, it is not possible to correctly evaluate mechanical properties of materials such as organics of which deformation mode has a high tendency of elastic deformation as well. Accordingly, the mechanical properties of the film composed of organics have to be evaluated in consideration of various properties.
In one conventional art in which properties of a surface layer of an electrophotographic photoreceptor having an organic photosensitive layer are evaluated, application of a universal hardness value (Hu) through a universal hardness test as defined in DIN50359-1 and a plastic deformation ratio (elasticity deformation ratio) is proposed ( e.g., refer to Japanese Unexamined Patent Publication JP-A 2000-10320).
An art described in JP-A 2000-10320 discloses that limitation of the universal hardness value (Hu) and the plastic deformation ratio to fall within a specific range makes mechanical deterioration of a photoreceptor surface layer less easily occur. However, the limited range of elasticity disclosed in JP-A 2000-10320 includes on a present condition substantially all of the photoreceptors having the charge transporting layer in which commonly used polymer binder is used and thus, there is a problem that a preferable range is not substantially limited.
Further, in the art described in JP-A 2000-10320, Hu and the plastic deformation ratio of the charge transporting layer serving as a surface layer are controlled by arranging kind and blending amount of the binder resin. However, there arises a problem that depending on the kind and blending amount of the binder resin, the sensitivity and light responsiveness of the photoreceptor may decrease.
Since the sensitivity and light responsiveness of the photoreceptor depend on the charge transporting ability of the charge transporting substance as described above, it is considered that a charge transporting substance of high charge transporting ability is used in order to suppress lowering of the sensitivity and light responsiveness. However, the charge transporting ability of the enamine compound as disclosed in JP-A 2-51162, JP-A 6-43674 or JP-A 10-69107 is insufficient and no sufficient sensitivity and light responsiveness can be obtained even by the use of the enamine compounds. Further, as in the photoreceptor disclosed in JP-A 7-134430, it can be considered to incorporate a polysilane and an enamine compound having a specified structure. However, a photoreceptor using the polysilane is sensible to light exposure, and brings about another problem of lowering the various characteristics as the photoreceptor when exposed to light, for example, during maintenance.
In other words, even if the charge transporting substance described in JP-A 2-51162, JP-A 6-43674, JP-A 10-69107, or JP-A 7-134430 is used for the photoreceptor described in JP-A 2000-10320, it is not possible to realize a photoreceptor having both of the electric characteristics such as the sensitivity and the light responsiveness, and the durability to the mechanical external force.
Further, as characteristics of the photoreceptor, it is demanded that characteristics change little by fluctuation of the circumstance and that circumstantial stability is excellent, however, a photoreceptor having such characteristics has not been obtained.